Tcl_CreateCommand defines a new command in interp and associates
it with procedure proc such that whenever cmdName is
invoked as a Tcl command (via a call to Tcl_Eval) the Tcl interpreter
will call proc to process the command.
It differs from Tcl_CreateObjCommand in that a new string-based
command is defined;
that is, a command procedure is defined that takes an array of
argument strings instead of objects.
The object-based command procedures registered by Tcl_CreateObjCommand
can execute significantly faster than the string-based command procedures
defined by Tcl_CreateCommand.
This is because they take Tcl objects as arguments
and those objects can retain an internal representation that
can be manipulated more efficiently.
Also, Tcl's interpreter now uses objects internally.
In order to invoke a string-based command procedure
registered by Tcl_CreateCommand,
it must generate and fetch a string representation
from each argument object before the call
and create a new Tcl object to hold the string result returned by the
string-based command procedure.
New commands should be defined using Tcl_CreateObjCommand.
We support Tcl_CreateCommand for backwards compatibility.

Tcl_CreateCommand will delete an existing command cmdName,
if one is already associated with the interpreter.
It returns a token that may be used to refer
to the command in subsequent calls to Tcl_GetCommandName.
If cmdName contains any :: namespace qualifiers,
then the command is added to the specified namespace;
otherwise the command is added to the global namespace.
If Tcl_CreateCommand is called for an interpreter that is in
the process of being deleted, then it does not create a new command
and it returns NULL.
Proc should have arguments and result that match the type
Tcl_CmdProc:

When proc is invoked the clientData and interp
parameters will be copies of the clientData and interp
arguments given to Tcl_CreateCommand.
Typically, clientData points to an application-specific
data structure that describes what to do when the command procedure
is invoked. Argc and argv describe the arguments to
the command, argc giving the number of arguments (including
the command name) and argv giving the values of the arguments
as strings. The argv array will contain argc+1 values;
the first argc values point to the argument strings, and the
last value is NULL.
Note that the argument strings should not be modified as they may
point to constant strings or may be shared with other parts of the
interpreter.

Note that the argument strings are encoded in normalized UTF-8 since
version 8.1 of Tcl.

Proc must return an integer code that is expected to be one of
TCL_OK, TCL_ERROR, TCL_RETURN, TCL_BREAK, or
TCL_CONTINUE. See the Tcl overview man page
for details on what these codes mean. Most normal commands will only
return TCL_OK or TCL_ERROR. In addition, proc must set
the interpreter result to point to a string value;
in the case of a TCL_OK return code this gives the result
of the command, and in the case of TCL_ERROR it gives an error message.
The Tcl_SetResult procedure provides an easy interface for setting
the return value; for complete details on how the interpreter result
field is managed, see the Tcl_Interp man page.
Before invoking a command procedure,
Tcl_Eval sets the interpreter result to point to an empty string,
so simple commands can return an empty result by doing nothing at all.

The contents of the argv array belong to Tcl and are not
guaranteed to persist once proc returns: proc should
not modify them, nor should it set the interpreter result to point
anywhere within the argv values.
Call Tcl_SetResult with status TCL_VOLATILE if you want
to return something from the argv array.

DeleteProc will be invoked when (if) cmdName is deleted.
This can occur through a call to Tcl_DeleteCommand or Tcl_DeleteInterp,
or by replacing cmdName in another call to Tcl_CreateCommand.
DeleteProc is invoked before the command is deleted, and gives the
application an opportunity to release any structures associated
with the command. DeleteProc should have arguments and
result that match the type Tcl_CmdDeleteProc:

typedef void Tcl_CmdDeleteProc(
ClientData clientData);

The clientData argument will be the same as the clientData
argument passed to Tcl_CreateCommand.